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SubjectRe: [PATCH] sched: avoid unnecessary overflow in sched_clock
On 11/15/2011 10:41 PM, Mike Galbraith wrote:
> On Tue, 2011-11-15 at 15:02 -0800, john stultz wrote:
>> On Tue, 2011-11-15 at 14:12 -0800, Salman Qazi wrote:
>>> (Added the missing signed-off-by line)
>>>
>>> In hundreds of days, the __cycles_2_ns calculation in sched_clock
>>> has an overflow. cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing
>>> the final value to become zero. We can solve this without losing
>>> any precision.
>>>
>>> We can decompose TSC into quotient and remainder of division by the
>>> scale factor, and then use this to convert TSC into nanoseconds.
>>>
>>> Signed-off-by: Salman Qazi<sqazi@google.com>
>>
>> Acked-by: John Stultz<johnstul@us.ibm.com>
>
> This wants a stable tag, no?
>
> -Mike
>

Probably a good idea -- This especially sucks rocks in the sched_clock_stable==1
case; resulting in it coming straight back out of sched_clock_cpu() and trashing
rq->clock.

- Paul

>>> ---
>>> arch/x86/include/asm/timer.h | 23 ++++++++++++++++++++++-
>>> 1 files changed, 22 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
>>> index fa7b917..431793e 100644
>>> --- a/arch/x86/include/asm/timer.h
>>> +++ b/arch/x86/include/asm/timer.h
>>> @@ -32,6 +32,22 @@ extern int no_timer_check;
>>> * (mathieu.desnoyers@polymtl.ca)
>>> *
>>> * -johnstul@us.ibm.com "math is hard, lets go shopping!"
>>> + *
>>> + * In:
>>> + *
>>> + * ns = cycles * cyc2ns_scale / SC
>>> + *
>>> + * Although we may still have enough bits to store the value of ns,
>>> + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
>>> + * leading to an incorrect result.
>>> + *
>>> + * To avoid this, we can decompose 'cycles' into quotient and remainder
>>> + * of division by SC. Then,
>>> + *
>>> + * ns = (quot * SC + rem) * cyc2ns_scale / SC
>>> + * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
>>> + *
>>> + * - sqazi@google.com
>>> */
>>>
>>> DECLARE_PER_CPU(unsigned long, cyc2ns);
>>> @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
>>>
>>> static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
>>> {
>>> + unsigned long long quot;
>>> + unsigned long long rem;
>>> int cpu = smp_processor_id();
>>> unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
>>> - ns += cyc * per_cpu(cyc2ns, cpu)>> CYC2NS_SCALE_FACTOR;
>>> + quot = (cyc>> CYC2NS_SCALE_FACTOR);
>>> + rem = cyc& ((1ULL<< CYC2NS_SCALE_FACTOR) - 1);
>>> + ns += quot * per_cpu(cyc2ns, cpu) +
>>> + ((rem * per_cpu(cyc2ns, cpu))>> CYC2NS_SCALE_FACTOR);
>>> return ns;
>>> }
>>>
>>>
>>
>>
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